The present invention relates to inhibited curable organopolysiloxane resins.
Organopolysiloxane resins in which the substituent organic groups are methyl, phenyl, vinyl and/or similar organic groups are well-known. These materials cure through a variety of mechanisms, such as peroxide catalyzed vinyl groups on silicon or by the platinum catalyzed addition of silicon hydride to alkenyl groups on silicon. The properties of these organopolysiloxane resins such as toughness and retention of strength at high temperatures make them useful in casting resin applications. The resins that are useful for casting applications are those that have higher viscosities, on the order of 5.0 to 200 Pa.s or higher. These resins, even though they have the desired strengths, have one important disadvantage. Because of their high viscosities they are extremely difficult to handle in use. They do not pour rapidly and have slow flow when cast into place.
This disadvantage was recently overcome by the discovery that certain reactive diluents could be added to these resins to give the desired pourability and flow characteristics. This technology has been incorporated in a patent application and filed under U.S. Pat. Ser. No. 912,794, filed June 5, 1978 in the name of Gary E. LeGrow which subject matter is incorporated herein by reference.
Additional problems arise when these resins are used for encapsulating delicate electronic equipment. Part of the advantage in using these resins is the fact that they are highly transparent in the cured state which allows easy visual inspection of the encapsulated electronic part. If the resins do not remain transparent or if they contain striations then the visual inspection of such parts is made more strenuous. Secondly, these resins, when all the reactants are mixed together, begin to cure in a manner such that there is a limited working time for use of the resin for encapsulation and thus, it would be highly desirable to be able to prolong the working time of the resins so as to prevent waste, maintain adequate physical properties of the cured resin and get reproducibility in the cured product.
Several systems have been suggested and patented for causing inhibition in platinum catalyzed, silicon hydride addition to unsaturated organic group reactions. Such systems are shown in, for example, U.S. Pat. Nos. 3,188,299 (pyridines, picolines); 3,188,300 (organophosphorus); 3,192,181 (benzotriazoles); 3,344, 111 (nitrile compounds); 3,383,356 (halocarbons); 3,445,420 (acetylenic compounds); 3,453,234 (sulfoxides); 3,532,649 (salts of tin, mercury, bismuth, copper) and 3,723,567 (aminoalkylalkoxysilanes).
Three of these patents show the use of amines to inhibit the platinum catalyzed silicon hydride addition reaction. U.S. Pat. No. 3,723,567 deals with primary and secondary amino compounds which it has been found do not work in the instant invention. U.S. Pat. Nos. 3,188,299 and 3,453,233 both deal with tertiary amino compounds but U.S. Pat. No. 3,188,299 deals with the inhibition of lower viscosity siloxanes and therefore, there is no striation problem to contend with as is the case when high viscosity resins are used. Furthermore, these materials are aromatic heterocyclic nitrogen compounds and bear no relationship to the inhibitors of the instant invention.
Finally U.S. Pat. No. 3,453,233 deals with the use of silazanes to inhibit the reaction of platinum catalyzed reactions. These silazanes, it is disclosed, also become part of the reaction and are incorporated into the final cured product. This differs from the instant invention which depends on the removal at elevated temperatures of the inhibitor compound.